Network Layer

1. OSI 7-Layer Model

Model Characteristics

• Network standard specification established by ISO

• Enables common network usage regardless of computer manufacturer

• Each layer is independent, so it's not affected by other layers during data transmission process

Characteristics of Each Layer

Layer 1 (Physical Layer)

• Controls physical connections between systems and converts electrical signals

• e.g. Convert signals to digital or analog

Layer 2 (Data Link Layer)

• Determines data transmission between network devices and physical addresses

• e.g. Bridge/switch, MAC address

Layer 3 (Network Layer)

• Responsible for data routing between actual networks and selects optimal paths

• e.g. IP packet transmission

Layer 4 (Transport Layer)

• Provides services to enable reliable data exchange between computers

• e.g. TCP/UDP connections

Layer 5 (Session Layer)

• Functions such as setting and releasing session connections, transmitting session messages

• In other words, determines communication methods between computers

Layer 6 (Presentation Layer)

• Encodes or decodes data passed to or received from application layer

• e.g. Data conversion such as character codes, compression, encryption

Layer 7 (Application Layer)

• Layer that ultimately provides interface with users

• Application programs executed by users

• e.g. File transfer, website viewing, etc.

Data Transmission Side

• Data transmission side passes data from upper layers to lower layers

• Each layer adds necessary information to data, and this information is called 'header' or 'trailer'

• Adding headers is called 'encapsulation'

Data Receiving Side

• Receives data passed through each layer from lower layers to upper layers

• Passing data to upper layers and removing headers at each layer is called 'de-encapsulation'

• After de-encapsulation, when reaching the final application layer, only the original data intended to be transmitted remains

2. TCP/IP 4-Layer Model

Model Characteristics

• Model simplified and made practical for real-world use based on OSI model

Characteristics of Each Layer

Layer 1 (Network Interface Layer)

• Corresponds to physical layer and data link layer of OSI layers

• Uses MAC as physical address

• LAN, packet network, etc.

Layer 2 (Internet Layer)

• Corresponds to network layer of OSI layers

• Responsible for transmitting IP packets between communication nodes and routing

• e.g. IP, ARP, RARP

Layer 3 (Transport Layer)

• Corresponds to transport layer of OSI layers

• Controls connections between communication nodes and is responsible for reliable data transmission

• e.g. TCP, UDP

Layer 4 (Application Layer)

• Corresponds to session, presentation, and application layers of OSI layers

• Used when implementing applications based on TCP/UDP

• e.g. FTP, HTTP, SSH